Apparatus for connecting a Stirling engine driven alternator to an ac mains supply is disclosed in our International Patent Application No. PCT/GB01/00840 and a modification thereof is disclosed in our U.K. Patent Application No. 0402587.0. FIG. 1 of the former application is reproduced herein as FIG. 1 and shows a linear alternator 10 that is driven by a Stirling engine (not shown). The mains supply is shown at 20 between neutral 21 and live 22. The alternator 10 is connected between neutral 21 and live 22 through two parallel paths 30 and 40. The first electrical path 30 has a switch 31 and a meter 32 in series. The second electrical path 40 has a first impedance 41, a second impedance 42, a switch 43 and a meter 44 connected in series in that order. A line 50 extends between impedances 41 and 42 to connect to neutral 21 via two switches 51 and 52.
Connection to the mains supply 20 is achieved through a sequence of stages corresponding to various arrangements of the switches 31, 43, 51 and 52. In particular, the switch 43 allows the alternator 10 to be connected to the mains supply 20 initially through impedances 41 and 42 thereby limiting the current passing through the alternator 10. This ensures that an appropriate force is exerted on the piston of the Stirling engine to initiate its stroke, i.e. a sufficient force is provided to initiate the movement but that is not so large as to cause the piston to be driven into the walls of the piston chamber. The engine may then be connected directly to the mains supply 20 through path 30. The alternator 20 is disconnected from the mains supply only after the alternator 10 is stalled by placing impedance 41 across live 22 and neutral 21.
The mains supply 20 is used to initiate the piston stroke of the Stirling engine, and so the Stirling engine should be in an operational condition to be able to maintain the reciprocating motion of the piston upon connection of the alternator 10 to the mains supply 20. Specifically, a burner of the Stirling engine is fired to heat one end of the piston cylinder and to raise the temperature of the Stirling engine prior to connection to the mains supply 20. There is an optimum time to connect the alternator 10 to the mains supply that is related to the operational condition of the Stirling engine.
When Stirling engine operation is no longer required, the burner is extinguished and the Stirling engine is allowed to run down. The alternator 10 is disconnected from the mains supply 20 during the running down process and, again, there is an optimum time for doing so.
It is an object of the present invention to improve on this background and, in particular, to provide connection to and disconnection from the mains supply with improved timing.